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Snakes in the Grass, Snakes in the Trees

Snakes are difficult to study in the field because of their secretive behavior, and many features of their ecology and natural history are not well known. Although snakes are frequently considered vermin and often indiscriminately killed by humans, most species of snakes are harmless to humans, and many serve in beneficial roles as predators of small mammals and invertebrates. Because many species of snakes are declining in numbers in regions where the land is dominated by row-crop agriculture, such as in central Illinois, it is important to identify and protect critical habitats and landscape features relevant to their conservation. For example, sites used for hibernation may be particularly important for snakes living in temperate climates. Conservation biologists also have recently begun to suspect that some species of snakes may be major predators of the eggs and nestlings of songbirds. Studies of habitat use by snakes could help resource managers determine if local manipulations of habitat could improve nesting success for declining species of songbirds.

We used radiotelemetry to monitor habitat use by black rat snakes (Elaphe obsoleta), fox snakes (Elaphe vulpina), and blue racers (Coluber constrictor)at the Middle Fork Fish and Wildlife Area (MFFWA) in Vermilion County, Illinois. In Illinois, black rat snakes may grow to about 68 inches long, fox snakes may grow to about 51 inches, and blue racers may grow to about 60 inches. All three species feed on a variety of small mammals, and may prey on nests of birds. Blue racers have the most general diet, which includes a variety of arthropods, annelids, amphibians, and reptiles.

Because these three species are similar in size and diet, we wanted to see if they differed in habitat use.

A blackrat snake, Elaphe obsoleta.

You can't put a radiocollar on a snake. Instead, we brought each snake that we captured to a surgical facility at the University of Illinois where, with the help of professional veterinarians, we anesthetized it with isoflurane gas and inserted a small radio transmitter into the body cavity. A flexible, slender wire antenna was inserted under the scales but above the muscle layer along the side of the snake. Thus, the body of the snake became its own broadcast antenna. We monitored each snake for two days after surgery to make sure of a healthy recovery, then released it where it was initially captured. In all, we implanted transmitters into 7 snakes (3 black rat snakes, 3 fox snakes, 1 blue racer) in 1997 and 17 snakes (10 black rat snakes, 2 fox snakes, 5 blue racers) in 1998.

We tried to locate each snake about three times per week during the spring and early summer, then about once per week in the late summer and fall until they moved to their hibernation sites. Each time we located a snake, we recorded the habitat in which the snake occurred, and whether the snake was below ground, on the soil surface, or above ground in vegetation. We also plotted the locations of each snake on maps of the MFFWA, and used a geographic information system (GIS) to conduct spatial analyses. These analyses allowed us to compare use of a type of habitat to the amount of that habitat available to each snake, and thus statistically evaluate selection and avoidance of habitat types.

Black rat snakes at the MFFWA were most frequently located in forested habitat. In contrast, fox snakes and blue racers both were typically found in prairies and old fields, and avoided forests. All three species generally avoided agricultural fields. Another interesting pattern to emerge involved the use of vertical space. Almost 50% of all the locations of black rat snakes were high up in trees, and an additional 15% of locations were in vegetation but below about 10 feet above the ground. The remaining 35% of locations were on the soil surface or below ground, most likely in rodent burrows. In contrast, all locations of fox snakes were on the surface or below ground. Blue racers were similar to fox snakes in their use of vertical space, except that about 5% of the locations were in vegetation less than 10 feet above the ground. Our telemetry data support the general descriptions given by Philip W. Smith, a former scientist with the Natural History Survey, in his 1961 bulletin The Amphibians and Reptiles of Illinois, and they provide the first quantitative data on habitat selection for these species in Illinois.

Snake being anesthetized by veterinarian at Univeristy of Illinois.

Although we do not know whether competition for resources influences the behavior of these species, it is interesting to speculate about how differences between species might facilitate their ability to coexist. Our data provide an example of two species in the same genus, black rat snakes and fox snakes, that have similar diets but differ strongly in their use of habitat where they occur together. In contrast, habitat use by fox snakes and blue racers was very similar at the level we examined it. Blue racers seemed to be active on the ground at warmer temperatures, to move more quickly at warmer temperatures, and to move more slowly at cooler temperatures than did fox snakes. Perhaps these species have different thermal preferences. Hatchling fox snakes readily feed on newborn mice whereas hatchling racers refuse mice and prefer insects. Blue racers also do not constrict their prey as much as black rat snakes and fox snakes do, and may be more limited in the size of the prey they can overpower. Perhaps differences in diet reduce competition between these species. The community ecology of snakes would make a fascinating subject for future studies.

We located three hibernation sites, including one site that appeared to be a major center for hibernating snakes. At least five species hibernated at this site, with black rat snakes aggregating there in large numbers. Some snakes migrated almost a mile from this site in the spring to their summer foraging areas, and returned in the fall. We prefer to keep the locations of these sites confidential, as not everyone appreciates snakes for the wonderful creatures they are.

The next time you're out walking in the early spring before the trees leaf out, you might look up to watch for a snake basking in the branches as well as watch out for the ones underfoot.

This project was supported by a grant from the Illinois Wildlife Preservation Fund and Federal Aid to Wildlife Restoration Grant W 125 through the Illinois Department of Natural Resources.